Magnetic powder for magnetic recording, magnetic recording medium, and method of manufacturing magnetic powder for magnetic recording

ABSTRACT

An aspect of the present invention relates to magnetic powder, which is magnetoplumbite hexagonal strontium ferrite magnetic powder comprising 0.05 atomic percent to 3 atomic percent of Ca per 100 atomic percent of Fe, but comprising no rare earth elements or transition metal elements other than Fe, the average particle size of which ranges from 10 nm to 25 nm, and which is magnetic powder for magnetic recording.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2013-272724 filed on Dec. 27, 2013. The aboveapplication is hereby expressly incorporated by reference, in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetic powder for magnetic recording.

The present invention further relates to a magnetic recording mediumcomprising the above magnetic powder for magnetic recording asferromagnetic powder in a magnetic layer, and to a method ofmanufacturing the above magnetic powder for magnetic recording.

2. Discussion of the Background

Hexagonal ferrite is widely employed as magnetic powder for magneticrecording. The coercive force thereof is great enough for use inpermanent magnetic materials. The magnetic anisotropy that is the basisof the coercive force derives from its crystal structure. Thus, highcoercive force can be maintained even when the size of the particles isreduced. Further, magnetic recording media employing hexagonal ferritemagnetic powder in a magnetic layer have high density characteristicsdue to the vertical component. Thus, hexagonal ferrite is ferromagneticpowder that is suited to high density recording.

In recent years, recording densities have been increasing in the fieldof magnetic recording. To achieve an accompanying reduction in noise,there has been a need to reduce the particle size of hexagonal ferrite.

However, when the size of hexagonal ferrite magnetic particles isreduced, the energy maintaining the magnetic particles in the directionof magnetization (magnetic energy) tends to be difficult to resistthermal energy. So-called thermal fluctuation ends up causing recordingretention property to drop, and the phenomenon whereby magnetic energyis overcome by thermal energy and recording is lost can no longer beignored. This point will be described in greater detail. “KuV/kT” is aknown index relating to the thermal stability of magnetization. Ku isthe anisotropy constant of a magnetic material, V is the particle volume(activation volume), k is the Boltzmann constant, and T is absolutetemperature. When the magnetic energy KuV is increased relative to thethermal energy kT, it is possible to inhibit the effect of thermalfluctuation. However, the particle diameter V, that is, the particlesize of the magnetic material, should be kept low to reduce the noise ofthe medium, as set forth above. Since the magnetic energy is the productof Ku and V, as stated above, it suffices to increase Ku to increase themagnetic energy when V is in the low range. However, the relationHK=2Ku/Ms exists between Ku and the anisotropy field HK. When Ku isincreased without a change in Ms, HK also increases. The anisotropyfield HK is a magnetic field intensity that is necessary to achievesaturation magnetization from the direction of the hard axis ofmagnetization. When HK is high, the reversal of magnetization by themagnetic head tends not to occur, recording (the writing of information)becomes difficult, and the reproduction output ends up dropping. Thatis, the higher the Ku of the magnetic particle, the more difficult it isto write information.

As set forth above, it is extremely difficult to satisfy all threecharacteristics of higher density recording, thermal stability, and easeof writing. This is known as the trilemma of magnetic recording. It willbe a major problem in achieving higher density recording in the future.

On the other hand, barium ferrite is widely employed as hexagonalferrite for use in magnetic recording. However, strontium ferrite isknown to have a higher Ku and a higher σs than barium ferrite. SinceHK=2Ku/Ms and Ms=σs×ρ (ρ: specific gravity), strontium ferrite is amagnetic material that is advantageous for resolving the trilemma byachieving a low HK with a high Ku. In this regard, Japanese UnexaminedPatent Publication (KOKAI) No. 2013-211316 or English language familymember US2013/256584A1, which are expressly incorporated herein byreference in their entirety, proposes a method of manufacturingstrontium ferrite that is suitable as magnetic powder for high-densityrecording.

SUMMARY OF THE INVENTION

As set forth above, strontium ferrite is a magnetic material that isuseful for resolving the trilemma. Accordingly, the present inventorconducted extensive research into the use of strontium ferrite asmagnetic powder for magnetic recording. As a result, he determined thatalthough strontium ferrite had a higher Ku than barium ferrite, Ku couldbe decreased and thermal stability tended to drop when in the form offine particles.

An aspect of the present invention provides for means for achieving bothenhanced thermal stability and a reduced size of particles of strontiumferrite that are employed as magnetic powder in magnetic recording.

Magnetoplumbite-type (also described as “M-type”), W-type, Y-type, andZ-type crystal structures of hexagonal ferrite are known. In extensiveresearch, the present inventor employed strontium ferrite of M-typecrystal structure. As set forth above, hexagonal ferrite is aferromagnetic material with magnetic characteristics that are suited toachieving higher density recording. Among the hexagonal ferrites, themagnetic characteristics of the M-type are advantageous for achievinghigher density recording. The present inventor conducted extensiveresearch in this regard. As a result, he discovered that by adding aprescribed quantity of calcium to M-type hexagonal strontium ferrite, itwas possible to avoid a reduction in Ku due to the formation of fineparticles. It thus became possible to provide M-type hexagonal strontiumin the form of fine particles with good thermal stability and a high Ku.Additionally, the incorporation of a prescribed quantity of calcium inM-type hexagonal strontium ferrite was also unexpectedly found to raisethe saturation magnetization σs.

However, the present inventor excluded the rare earth elements andtransition metal elements other than Fe from the elements contained inthe calcium-containing M-type hexagonal strontium ferrite. This was donefor the following reasons.

M-type hexagonal ferrite has a structure denoted by the compositionalformula MFe₁₂O₁₉. Strontium ferrite primarily contains strontium as thealkaline earth metal element denoted by M. Calcium ferrite, whichcontains primarily calcium, is also known. Because the order of thealkaline earth metals when arranged by magnitude of ion radius isBa>Sr>Ca, when calcium ferrite contains no substitute elements, itcannot assume an M-type crystal structure. Thus, calcium ferritesubstitute elements in the form of rare earth elements such as La andtransition metal elements such as Co have been incorporated into calciumferrite to form an M-type crystal structure (for example, see JapaneseUnexamined Patent Publication (KOKAI) No. 2010-1171, which is expresslyincorporated herein by reference in its entirety). However, the presenceof rare earth elements and transition metal elements (excluding the Fethat is required to form ferrite) would impart variation to thecompositional distribution between particles. As a result, the ratioaccounted for by particles with poor recording retention property(particles that end of quickly demagnetizing after being recorded) andparticles of poor magnetic characteristics that do not contribute torecording increases. This would compromise the magnetic characteristicdistribution. Specifically, the switching field distribution (SFD) wouldrise and the electromagnetic characteristics would end up beingcompromised.

Accordingly, the present inventor excluded rare earth elements andtransition metal elements other than Fe from the elements contained inM-type hexagonal strontium ferrite. In the present invention, the term“rare earth elements” refers to Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb, and Lu. The term “transition metal elements otherthan Fe” refers to Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Hf,Ta, and W. Since strontium has been adopted as the primary alkalineearth metal element constituting hexagonal ferrite, it is possible toadopt an M-type crystal structure in which no rare earth elements ortransition metal elements other than Fe are present. It is thus possibleto provide M-type hexagonal strontium ferrite that is suitable asmagnetic powder for magnetic recording and has good thermal stability,and with a low SFD.

The present invention was devised on the basis of the above discoveries.

An aspect of the present invention relates to magnetic powder, which ismagnetoplumbite (also referred to as magnetoplumbite-type or M-type)hexagonal strontium ferrite magnetic powder comprising 0.05 atomicpercent to 3 atomic percent of Ca per 100 atomic percent of Fe, butcomprising no rare earth elements or transition metal elements otherthan Fe, the average particle size of which ranges from 10 nm to 25 nm,and which is magnetic powder for magnetic recording.

In the present invention, the average particle size refers to the valuedetermined by the method set forth below.

A photograph is taken of the particles at a magnification of100,000-fold with a model H-9000 transmission electron microscope madeby Hitachi, and printed on print paper at a total magnification of500,000-fold. Target particles are selected from the photograph of theparticles, the contours of the particles are traced with a digitizer,and Carl Zeiss image analysis software KS-400 is used to measure thesize of the particles. The particle sizes of 500 particles aredetermined. In this context, the particle size is a primary particlesize. The term “primary particle” refers to an unaggregated, independentparticle. The average value (arithmetic average) of the particle size ofthe 500 particles obtained in this manner is adopted as the averageparticle size of the powder.

In the present invention, the size of the particles constituting powderis denoted as follows based on the shape of the particles observed inthe above particle photograph:

-   (1) When acicular, spindle-shaped, or columnar (with the height    being greater than the maximum diameter of the bottom surface) in    shape, the particle size is denoted as the length of the major axis    constituting the particle, that is, the major axis length.-   (2) When platelike or columnar (with the thickness or height being    smaller than the maximum diameter of the plate surface or bottom    surface) in shape, the particle size is denoted as the maximum    diameter of the plate surface or bottom surface.-   (3) When spherical, polyhedral, of unspecific shape, or the like,    and the major axis constituting the particle cannot be specified    from the shape, the particle size is denoted as the diameter of an    equivalent circle. The term “diameter of an equivalent circle” means    that obtained by the circle projection method.

When the particle has a specific shape, such as in the particle sizedefinition of (1) above, the average particle size is the average majoraxis length. In the case of (2), the average particle size is theaverage plate diameter, with the average plate ratio being thearithmetic average of (maximum diameter/thickness or height). For thedefinition of (3), the average particle size is the average diameter(also called the average particle diameter).

The average particle size set forth above can be obtained by observingthe powder that is present as powder by a transmission electronmicroscope. A measurement sample of the powder that is contained in themagnetic layer of a magnetic recording medium can be obtained bycollecting powder from the magnetic layer. The measurement sample can becollected, for example, by the following method.

1. Subjecting the surface of the magnetic layer to 1 to 2 minutes ofsurface treatment with a plasma reactor made by Yamato Scientific Co.,Ltd., and ashing the organic components (binder component and the like)of the surface of the magnetic layer to remove them.

2. Adhering filter paper that has been immersed in an organic solventsuch as cyclohexanone or acetone to the edge portion of a metal rod,rubbing the surface of the magnetic layer that has been treated as in 1.above on it, and transferring the magnetic layer component from themagnetic tape to the filter paper to separate it.

3. Shaking off the component separated by 2. above in a solvent such ascyclohexanone or acetone (placing each piece of filter paper in solventand using an ultrasonic disperser to shake it off), drying the solvent,and removing the separated component.

4. Placing the component that has been scraped off in 3. above in aglass test tube that has been thoroughly cleaned, adding n-butyl amineto about 20 mL of the magnetic layer component, and sealing the glasstest tube. (The n-butyl amine is added in a quantity adequate todecompose the remaining binder that has not been ashed.)

5. The glass test tube is heated for equal to or more than 20 hours at170° C. to decompose the binder and curing agent component.

6. The precipitate following the decomposition of 5. above is thoroughlywashed with pure water and dried, and the powder is recovered.

7. A neodymium magnet is placed near the powder that has been collectedin 6. and the powder that is attracted (that is, magnetic powder) iscollected.

Magnetic powder can be collected from the magnetic layer by the abovesteps. Since the above processing can impart almost no damage to theparticles, the above method permits measurement of the particle size ofpowder in the state in which it was contained in the magnetic layer.

In an embodiment, in the above magnetic powder, the content of Ca per100 atomic percent of the combined content of Sr and Ca ranges from 1atomic percent to 20 atomic percent.

In an embodiment, the above magnetic powder further comprises Al.

In an embodiment, in the above magnetic powder, the content of Al per100 atomic percent of Fe ranges from 0.5 atomic percent to 6 atomicpercent.

In an embodiment, in the above magnetic powder, at least a part of theAl is present on the surface of the magnetic powder.

In an embodiment, the above magnetic powder further comprises Ba.

In an embodiment, in the above magnetic powder, the content of Ba per100 atomic percent of the combined content of Sr, Ca, and Ba ranges from5 atomic percent to 40 atomic percent.

A further aspect of the present invention relates to a magneticrecording medium, which comprises a magnetic layer comprisingferromagnetic powder and binder on a nonmagnetic support, wherein theferromagnetic powder is the above magnetic powder.

A further aspect of the present invention relates to a method ofmanufacturing magnetic powder for magnetic recording, which comprisesproviding the magnetic powder for magnetic recording set forth above bya glass crystallization method with a starting material mixturecomprising at least Sr, Ca, and Fe.

In an embodiment, the starting material mixture further comprises Al. ACa-containing M-type hexagonal strontium ferrite further containing Alcan be thus obtained.

In an embodiment, the starting material mixture further comprises Ba. ACa-containing M-type hexagonal strontium ferrite further containing Bacan be thus obtained.

A further aspect of the present invention relates to a method ofmanufacturing magnetic powder for magnetic recording, wherein themagnetic powder for magnetic recording medium is the magnetic powder formagnetic recording set forth above, and the method comprises:

preparing a hexagonal ferrite precursor by mixing an Fe salt, an Srsalt, and a Ca salt in a base-containing water-based solution; and

converting the hexagonal ferrite precursor to hexagonal ferrite byfeeding a water-based solution comprising the hexagonal ferriteprecursor that has been prepared continuously to a reaction flow passagewhile heating the water-based solution to a temperature of equal to orhigher than 300° C. as well as applying a pressure of equal to or higherthan 20 MPa to the water-based solution.

In an embodiment, the conversion to hexagonal ferrite is conducted by:

mixing the water-based solution comprising the hexagonal ferriteprecursor that has been prepared with an organic modifying agent; then

mixing a solution, that has been provided by the mixing, with water thatis being continuously fed while heating and applying pressure to preparea mixture and feeding the mixture to the reaction flow passage.

In an embodiment, the above method further comprises heating andpressurizing a water-based solution comprising an Al compound andhexagonal ferrite that has been provided by the conversion of thehexagonal ferrite precursor to make Al adhere to a surface of thehexagonal ferrite.

In an embodiment, the heating and pressurizing is conducted by feedingthe water-based solution, comprising an Al compound and hexagonalferrite that has been prepared by the conversion of the hexagonalferrite precursor, continuously to a reaction flow passage the fluidflowing through which is heated to equal to or higher than 300° C. andpressurized to a pressure of equal to or higher than 20 MPa.

In an embodiment, the above method further comprises mixing thewater-based solution comprising an Al compound and hexagonal ferriteprovided by the conversion of the hexagonal ferrite precursor with waterthat is continuously fed while being heated and pressurized, and thenfeeding to the reaction flow passage.

In an embodiment, the above method further comprises mixing a Ba saltwith an Fe salt, Sr salt, and Ca salt during the mixing in abase-containing water-based solution. A Ca-containing M-type hexagonalstrontium ferrite containing Ba can be thus obtained.

An aspect of the present invention can provide M-type hexagonalstrontium ferrite in the form of fine particles, with a high Ku and goodthermal stability. Since no rare earth elements or transition metalelements other than Fe, which would cause variation in the magneticcharacteristic distribution, are contained, it is possible to achieve alow SFD.

It is also possible to provide magnetic powder for magnetic recordinghaving a high saturation magnetization σs by incorporating a prescribedquantity of calcium into M-type hexagonal strontium ferrite.

Using the above magnetic powder for magnetic recording as theferromagnetic powder in a magnetic layer can provide a magneticrecording medium that is suited to high-density recording.

Other exemplary embodiments and advantages of the present invention maybe ascertained by reviewing the present disclosure and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in the following text by theexemplary, non-limiting embodiments shown in the drawing, wherein:

FIG. 1 is a schematic diagram of a manufacturing device that is suitedto a continuous hydrothermal synthesis process.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Unless otherwise stated, a reference to a compound or component includesthe compound or component by itself, as well as in combination withother compounds or components, such as mixtures of compounds.

As used herein, the singular forms “a,” “an,” and “the” include theplural reference unless the context clearly dictates otherwise.

Except where otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by the present invention. Atthe very least, and not to be considered as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding conventions.

Additionally, the recitation of numerical ranges within thisspecification is considered to be a disclosure of all numerical valuesand ranges within that range. For example, if a range is from about 1 toabout 50, it is deemed to include, for example, 1, 7, 34, 46.1, 23.7, orany other value or range within the range.

The following preferred specific embodiments are, therefore, to beconstrued as merely illustrative, and non-limiting to the remainder ofthe disclosure in any way whatsoever. In this regard, no attempt is madeto show structural details of the present invention in more detail thanis necessary for fundamental understanding of the present invention; thedescription taken with the drawings making apparent to those skilled inthe art how several forms of the present invention may be embodied inpractice.

Magnetic Powder for Magnetic Recording

An aspect of the present invention relates to magnetic powder, which ismagnetoplumbite hexagonal strontium ferrite magnetic powder comprising0.05 atomic percent to 3 atomic percent of Ca per 100 atomic percent ofFe, but comprising no rare earth elements or transition metal elementsother than Fe, the average particle size of which ranges from 10 nm to25 nm, and which is magnetic powder for magnetic recording. For thereasons given above, no rare earth elements or transition metal elementsother than Fe are incorporated.

The magnetic powder for magnetic recording (also referred to simply asthe “magnetic power” hereinafter or the “magnetic recording powder”)will be described in greater detail below.

The average particle size of the above magnetic powder ranges from 10 nmto 25 nm. When the average particle size is less than 10 nm, thermalfluctuation may cause the stability of magnetization to decrease,compromising the SFD. At greater than 25 nm, noise may increase and theSNR may decrease. From the perspective of achieving both a higher SNRand a lower SFD, the average particle size desirably ranges from 10 nmto 23 nm, preferably within a range of 10 nm to 20 nm. The particle sizeof the magnetic powder can be adjusted by means of manufacturingconditions such as the crystallization conditions.

The above magnetic powder is M-type hexagonal strontium ferrite and theprimary alkaline earth metal element contained in the magnetic powder isSr. In this context, the term “primary alkaline earth metal element”refers to the alkaline earth metal element that accounts for thegreatest portion on an atom basis among the alkaline earth metalelements contained in the magnetic powder. The quantities of elementsand the proportions of elements in the magnetic powder can be determinedby elemental analysis by known methods.

As set forth above, the magnetic powder according to an aspect of thepresent invention is M-type strontium ferrite containing Ca. When thequantity of Ca is less than 0.05 atomic percent per 100 atomic percentof Fe, it becomes difficult to achieve a rise in Ku by adding Ca.Conversely, when the quantity of Ca exceeds 3 atomic percent per 100atomic percent of Fe, the proportion of the alkaline earth metalelements in the magnetic powder that is accounted for by Ca is high. Asa result, Ca, which has a small ion radius and tends not to beincorporated into an M-type crystal structure, ends up precipitating outas Ca salt, making it difficult to achieve an adequate effect in raisingthe Ku by adding Ca. Further, the precipitation of Ca salt may producevariation in the compositional distribution between particles, which mayultimately raise the SFD. Alternatively, unless a rare earth element ora transitional metal element other than Fe is present, it becomesdifficult to achieve an M-type crystal structure. However, the presenceof these elements ends up causing a rise in the SFD, as set forth above.For the reasons given above, the content of Ca in the magnetic powder isset to a range of 0.05 atomic percent to 3 atomic percent per 100 atomicpercent of Fe. From the perspectives of achieving a high Ku and a lowSFD, the Ca content is desirably kept to within a range of 0.1 atomicpercent to 3 atomic percent per 100 atomic percent of Fe, preferably towithin a range of 0.3 atomic percent to 3 atomic percent. From the sameperspective, the Ca content is desirably kept to within a range of 1atomic percent to 20 atomic percent per 100 atomic percent of thecombined content of Sr and Ca, preferably to within a range of 2 atomicpercent to 20 atomic percent, and more preferably, to within a range of3 atomic percent to 20 atomic percent. The method of manufacturingM-type hexagonal strontium ferrite containing a prescribed quantity ofCa will be described further below. In one embodiment of the abovemagnetic powder, Ca is contained at the position of a divalent elementdenoted by M in the crystal structure of M-type hexagonal ferritedenoted by the compositional formula MFe₁₂O₁₉. That is, it is containedin the M-type hexagonal strontium ferrite as a substitute element forSr. However, the state in which Ca is present is not limited to theabove; it can also be present in other states, such as a coating on thesurface of the particles of M-type hexagonal strontium ferrite.

The above magnetic powder contains alkaline earth metal elements in theform of Sr and Ca as essential components. However, it can also containanother alkaline earth metal element in the form of Ba. Theincorporation of Ba makes it possible to further lower the SFD. Thispoint was also discovered for the first time ever by the presentinventor. Details will be described below.

The SFD is an index of the distribution of magnetic characteristics. TheSFD rises as the variation in the composition between particlescontained in the magnetic powder increases. A large amount of variationin particle size between particles will also cause the SFD to rise.Accordingly, reducing the variation in particle size between particles,that is, achieving a sharp particle size distribution, is an effectiveway to further lower the SFD. However, as described in above-citedpublication, Japanese Unexamined Patent Publication (KOKAI) No.2013-211316, the particle size distribution of strontium ferrite tendsto be broader than that of barium ferrite. Japanese Unexamined PatentPublication (KOKAI) No. 2013-211316 proposes resolving this point byadjusting the composition of the starting material mixture in the courseof preparing strontium ferrite by the glass crystallization method. Incontrast, the present inventor discovered for the first time ever thatby adding a small quantity of Ba during the manufacturing of strontiumferrite, it was possible to obtain strontium ferrite with a sharpparticle size distribution. This was attributed by the present inventorto the fact that because the crystallization temperature of bariumferrite is lower than the crystallization temperature of strontiumferrite, the presence of a small quantity of Ba may cause barium ferriteto crystallize (precipitate) prior to the crystallization of thestrontium ferrite, with the barium ferrite serving as nuclei forcrystallization of the strontium ferrite.

From the perspective of achieving a good effect by means of the presenceof Ba as set forth above, in the magnetic powder according to an aspectof the present invention, the quantity of Ba that is incorporated isdesirably such that the Ba content is kept to within a range of 5 atomicpercent to 40 atomic percent per 100 atomic percent of the combinedcontent of Sr, Ca, and Ba. The Ba content is also desirably kept towithin a range of 1 atomic percent to 5 atomic percent per 100 atomicpercent of Fe.

The above magnetic powder contains Ca and can contain Ba as set forthabove. It can also further contain Al as an optional element. The statein which the Al is present is not specifically limited. It can becontained within the crystal structure of the M-type hexagonal ferrite,or can adhere to the surface of the hexagonal ferrite. In oneembodiment, at least a part of the Al adheres to the surface of thehexagonal ferrite. Al which adheres to the surface of the particle caninhibit aggregation between particles, thereby achieving a further risein the SNR. Accordingly, in one embodiment, the magnetic powder cancomprise a core of hexagonal ferrite (Ca-containing M-type hexagonalstrontium ferrite) and an Al-containing coating as a shell in acore/shell structure. The shell can be present on the core in acontinuous phase, or in a discontinuous phase where some portions arenot coated, known as an island-sea structure. By way of example, thecontent of Al in the magnetic powder ranges from 0.1 atomic percent to10 atomic percent per 100 atomic percent of Fe. From the perspective offurther raising the SNR, it is desirably equal to or higher than 0.5atomic percent. From the perspective of further raising the SNR andachieving a low SFD, the Al content is desirably equal to or less than 7atomic percent, preferably equal to or less than 6 atomic percent, per100 atomic percent of Fe. From the perspective of magneticcharacteristics, the greater the proportion of the particle that isaccounted for by the portion exhibiting magnetic properties, the better.From this perspective, the Al content is desirably equal to or less than7 atomic percent, preferably equal to or less than 6 atomic percent, per100 atomic percent of Fe. One embodiment of the method of manufacturingmagnetic powder containing Al will be set forth below.

The magnetic powder according to an aspect of the present inventioncontains no rare earth elements or transition metal elements other thanFe. It contains Sr, Fe, and 0 constituting M-type hexagonal ferrite, anda prescribed quantity of additional Ca. It can also contain optionalelements in the form of Ba and Al. In one embodiment, it can alsocontain other elements, so long as they are not rare earth elements ortransition metal elements (excluding Fe). Examples of the other elementsare Bi, Si and the like that can be added to adjust the shape of theparticles and the like. It is also possible to add elements that areknown for adjusting the coercive force of hexagonal ferrite.Alternatively, in one embodiment, the magnetic powder desirably does notcontain the other elements. In this context, the term “does not contain”means that none is actively added in the process of manufacturing as anelement that is contained in the magnetic powder, and is not intended toapply to impurities. For example, the inclusion of a quantity of aboutequal to or less than 0.001 atomic percent per 100 atomic percent of Feis permissible.

As set forth above, the M-type crystal structure is advantageous forachieving higher density recording among the various crystal structuresof hexagonal ferrite. Thus, the above magnetic powder will desirablyreveal a single phase magnetoplumbite structure by XRD diffractionanalysis (X-ray diffraction analysis). The XRD diffraction analysis canbe conducted under the conditions described in Examples further below,for example.

Method of Manufactoring Magnetic Powder for Magnetic Recording

The magnetic powder for magnetic recording according to an aspect of thepresent invention can be manufactured by any manufacturing method andthe manufacturing method thereof is not specifically limited. By way ofexample, a method that is known as a method of manufacturing M-typehexagonal ferrite can be adopted as the manufacturing method, such asthe coprecipitation method, reverse micelle method, hydrothermalsynthesis method, and glass crystallization method. In the variousmanufacturing methods, Ca can be incorporated in the starting materialmixture in addition to Sr and Fe needed to obtain strontium ferrite toobtain Ca-containing M-type strontium ferrite.

It is said that the glass crystallization method is a good method formanufacturing hexagonal ferrite for magnetic recording because of pointssuch as yielding magnetic powder with fine particle suitability and/orsingle particle dispersion suitability that are desirable in a magneticrecording medium, with a narrow particle size distribution, and thelike. Accordingly, in one aspect of the present invention, the magneticpowder is desirably prepared by the glass crystallization method.Alternatively, the magnetic powder according to an aspect of the presentinvention is desirably obtained by a synthesis method based on thecontinuous hydrothermal synthesis process (also referred to hereinafteras the “supercritical synthesis method”) that has been proposed as amethod of synthesizing nanoparticles in recent years. Thesemanufacturing methods will be described in greater detail below.

A method of manufacturing hexagonal ferrite based on the glasscrystallization method generally comprises the following steps:

-   (1) The step of melting a starting material mixture comprising    hexagonal ferrite-forming components and glass-forming components to    obtain a melt (melting step);-   (2) The step of quenching the melt to obtain an amorphous material    (amorphous material-forming step);-   (3) The step of heat treating the amorphous material to cause    hexagonal ferrite particles to precipitate (crystallization step);    and-   (4) The step of collecting the hexagonal ferrite magnetic particles    that have precipitated from the heat-treated product (particle    collecting step).

By way of example, see Japanese Unexamined Patent Publication (KOKAI)No. 2011-213544, paragraphs 0018 to 0035 and Japanese Unexamined PatentPublication (KOKAI) No. 2011-225417, paragraphs 0013 to 0024 for detailsregarding the above steps. The above publications are expresslyincorporated herein by reference in their entirety. Normally, all ornearly all of the Ca that is contained in the starting material mixturewill be incorporated into the magnetic powder that is prepared. Thus,the Ca content of the magnetic powder obtained can be controlled bymeans of the composition of the starting material mixture. Accordingly,it is desirable to employ a starting material mixture containing Cafalling within a range of 0.05 atomic percent to 3 atomic percent per100 atomic percent of Fe. The Ca can be mixed in during preparation ofthe starting material mixture as an oxide, or as various salts (such ascarbonates) that will change into oxides in steps such as the meltingstep. As set forth above, the magnetic powder according to an aspect ofthe present invention can contain Ba. Since all or nearly all of the Bathat is contained in the starting material mixture will normally beincorporated into the magnetic powder that is prepared, the content ofBa in the magnetic powder that is obtained can be controlled by means ofthe composition of the starting material mixture. By way of example, theuse of a starting material mixture containing Ba falling within a rangeof 5 atomic percent to 40 atomic percent per 100 atomic percent of thecombined contents of Sr, Ca, and Ba will yield a magnetic powdercontaining that amount of Ba. In the same manner as Ca and the like, Bacan be mixed in during the preparation of the starting material mixturein the form of an oxide, or in the form of various salts (such ascarbonates) that will change into oxides in steps such as melting. Asset forth above, the present inventor presumes that the incorporation ofBa into the starting material mixture can cause barium ferrite tocrystallize (precipitate) prior to crystallization of strontium ferrite,with the barium ferrite serving as nuclei to obtain magnetic particlesexhibiting a sharp particle size distribution.

As described above, the magnetic powder according to an aspect of thepresent invention can contain Al. The method of adding Al is notspecifically limited. For example, it can be adhered to the surface ofthe magnetic powder by a surface treatment. In a desirable embodiment,Al is incorporated in the starting material mixture in the glasscrystallization method. As described in paragraph 0008 in JapaneseUnexamined Patent Publication (KOKAI) No. 2011-225417, Al can adhere tothe surface of the magnetic particles in a state of non-aggregation.Desirably, a starting material mixture containing Al in the form of anoxide, or a compound capable of changing into an oxide in the meltingstep or the like (such as a hydroxide) is employed in the glasscrystallization method. Reference can be made to the description inJapanese Unexamined Patent Publication (KOKAI) No. 2011-225417,paragraph 0017, for details on starting material mixtures containing Al.In one embodiment, a starting material mixture containing about 0.1 molepercent to about 10 mole percent of an Al compound can be employed.Reference can be made to the description in Japanese Unexamined PatentPublication (KOKAI) No. 2011-225417, paragraphs 0013 to 0024, and thelike for details on the embodiment of preparing magnetic powdercontaining Al by the glass crystallization method.

The manufacturing method based on a continuous hydrothermal synthesisprocess will be described next.

The manufacturing method based on a continuous hydrothermal synthesisprocess is a method by which a water-based fluid containing a hexagonalferrite precursor is heated and pressurized while being fed to areaction flow passage. The high reactivity of water at high temperatureand high pressure, desirably water in a subcritical to supercriticalstate, is used to convert the hexagonal ferrite precursor to ferrite.

The above hexagonal ferrite precursor can be obtained by mixing an Fesalt and an alkaline earth metal salt in a base-containing water-basedsolution. The magnetic powder according to an aspect of the presentinvention contains alkaline earth metals in the form of Sr and Ca, so ata minimum an Sr salt and a Ca salt are employed as alkaline earth metalsalts. By further employing a Ba salt, it is possible to contain amagnetic powder that also contains Ba. The quantity of each of the saltsemployed can be determined based on the desired magnetic powdercomposition taking into account the reaction rates of the variouselements, the reaction conditions in the continuous hydrothermalsynthesis process, and the like. The quantities of the various saltsemployed are desirably determined so that the contents of the variouselements fall within the above ranges or desirable ranges. Optionalpreliminary experiments can also be conducted to determine thequantities employed.

Normally, salts containing Fe and alkaline earth metals can precipitateas particles, desirably colloidal particles, in the base-containingwater-based solution. The particles that precipitate can be subsequentlyconverted to ferrite and become hexagonal ferrite magnetic particles byplacing them in the presence of water under high temperature and highpressure, desirably water in a subcritical to supercritical state.

A water-soluble salt is desirable as the alkaline earth metal salt. Forexample, hydroxides; halides such as chlorides, bromides, and iodides;and nitrates can be employed.

Fe salts in the form of water-soluble salts of iron such as halides suchas chlorides, bromides, and iodides; nitrates; sulfates; carbonates;organic acid salts; complex salts; and the like can be employed.

The above-described salts can be mixed in the base-containingwater-based solution to cause particles containing the elements thatwere contained in the salts (the hexagonal ferrite precursor) toprecipitate. The particles that precipitate can be subsequentlyconverted to ferrite and then hexagonal ferrite. In the presentinvention, the term “base” refers to one or more bases as defined byArrhenius, Bronsted, or Lewis (Arrhenius bases, Bronsted bases, or Lewisbases). Specific examples are sodium hydroxide, potassium hydroxide,sodium carbonate, and ammonia water. However, there is no limitation tothese bases. Nor is there any limitation to inorganic bases; organicbases can also be employed. The quantity of base that is employed in thewater-based solution is desirably about 0.1 to 10-fold based on weight,preferably about 0.2 to 8-fold, the combined weight of the salts addedto the water-based solution. The higher the concentration of the base,the finer the particles that precipitate tend to be. Since there arecases where salts contained in the water-based solution along with thebase will also be acid substances, the pH of the water-based solution isnot limited to being basic; there will be cases where it is neutral oracidic. By way of example, the pH of the water-based solution, as the pHat the temperature of the solution during the reaction, is equal to orhigher than 4 but equal to or lower than 14. From the perspective ofgetting the precursor synthesis reaction to proceed smoothly, it isdesirably equal to or higher than 5 but equal to or lower than 14,preferably equal to or higher than 6 but equal to or lower than 13, andmore preferably equal to or higher than 6 but equal to or lower than 12.A pH that is equal to or higher than 7 or exceeds 7 (neutral to basic)is still more desirable. Any acid can be used to adjust the pH. In thesame manner as for the base, the acid refers to one or more acids asdefined by Arrhenius, Bronsted, or Lewis (Arrhenius acids, Bronstedacids, or Lewis acids). Any of the acids commonly employed to adjust thepH can be employed without limitation as the acid. Specific examples arehydrochloric acid, nitric acid, and sulfuric acid. There is nolimitation to inorganic acids; organic acids can also be employed. Thesolution temperature of the above water-based solution can be controlledby heating or cooling, or can be room temperature without temperaturecontrol. The solution temperature desirably ranges from 10° C. to 90° C.Even when uncontrolled (for example, about 20° C. to 25° C.), thereaction will proceed adequately.

The solvent (water-based solvent) contained in the above water-basedsolution can be simply water, or can be a mixed solvent of water and anorganic solvent. The term “water-based solvent” refers to a solventcontaining water, desirably to a solvent containing equal to or morethan 50 weight percent of water per the total quantity of solvent. Thesolvent employed to prepare the precursor is preferably water alone.

An organic solvent that is miscible with water or hydrophilic isdesirable as a water-based solvent for use in combination with water.From this perspective, the use of a polar solvent is suitable. In thiscontext, the term “polar solvent” refers to a solvent with a dielectricconstant of equal to or higher than 15, a solubility parameter of equalto or higher than 8, or both. Examples of desirable organic solvents arealcohols, ketones, aldehydes, nitriles, lactams, oximes, amides, ureas,amines, sulfides, sulfoxides, phosphoric acid esters, carboxylic acids,and carboxylic acid derivatives in the form of esters, carbonic acid,carbonic acid esters, and ethers.

The hexagonal ferrite precursor can also be prepared in the presence ofan organic compound. A precursor that has been prepared in the presenceof an organic compound could conceivably be subjected to a conversionreaction to hexagonal ferrite in a state where the organic compoundadheres to the surface, and crystallized after having beeninstantaneously dissolved in a high-temperature, high-pressure system tocause hexagonal ferrite particles to precipitate (conversion tohexagonal ferrite). The presence of an organic compound in the vicinityof the particles from dissolution to crystallization is presumed by thepresent inventor to contribute to achieving the crystallization of finehexagonal ferrite particles of uniform particle size. The fact thatsynthesis in the presence of an organic compound can inhibit aggregationof the precursor and makes it possible to obtain a precursor in the formof fine particles with good uniformity of particle size is also thoughtto contribute to obtaining hexagonal ferrite in the form of fineparticles of good uniformity of particle size. The details of theorganic compound are as set forth below for the organic modifyingcompound. The organic compound need only be present in the reactionsolution along with the starting material compounds of the precursor;the mixing order with the starting material compounds of the precursoris not specifically limited. From the perspective of more effectivelypreventing aggregation of the precursor during preparation, the organiccompound is desirably added to the reaction solvent and dissolved orsuspended, after which the starting material compounds are added. Theorganic compound can be added as is to the reaction solvent, or added inthe form of a solution or suspension. Solvents that can be used insolutions or suspensions are as set forth above.

The quantity of organic solvent that is employed desirably ranges from0.01 weight part to 1,000 weight parts, preferably within a range of0.05 weight part to 500 weight parts, and more preferably, within arange of 0.1 weight part to 300 weight parts per 100 weight parts ofprecursor. The quantity of precursor serving as a basis here is themeasured value or the quantity that is theoretically produced from thequantities of starting materials charged. This also holds true for thevalues described below for the quantities based on the quantity ofprecursor.

Next, the water-based solution containing the hexagonal ferriteprecursor is heated and pressurized, and the water contained is placedin a state of high temperature and high pressure, preferably in asubcritical to supercritical state, to cause the hexagonal ferriteprecursor to undergo a ferrite conversion reaction (ferrite conversion)within the particles. As a result, hexagonal ferrite particles (M-typehexagonal strontium ferrite particles containing Ca) can be obtained.Generally, a fluid containing water as solvent is heated to equal to orhigher than 300° C. and subjected to a pressure of equal to or higherthan 20 MPa to place the water contained in the fluid in a subcriticalto supercritical state.

The following are examples of specific embodiments of the process ofconverting the hexagonal ferrite precursor to hexagonal ferrite.

(1) A water-based solution containing hexagonal ferrite precursor iscontinuously fed to a reaction flow passage that heats the fluid flowingthrough it to equal to or higher than 300° C. and pressurizes the fluidto a pressure of equal to or higher than 20 MPa, thereby converting thehexagonal ferrite precursor to hexagonal ferrite within the reactionflow passage.

(2) After the water-based solution containing the particles of hexagonalferrite precursor has been mixed with heated and pressurized water,desirably water that has been heated to equal to or higher than 200° C.and subjected to a pressure of equal to or higher than 20 MPa, it iscontinuously fed to a reaction flow passage that heats the fluid flowingthrough it to equal to or higher than 300° C. and subjects the fluid toa pressure of equal to or higher than 20 MPa, thereby converting thehexagonal ferrite precursor to hexagonal ferrite.

Embodiment (2) differs from embodiment (1) in that the heated andpressurized water is brought into contact with the water-based solutioncontaining the hexagonal ferrite precursor, while, in embodiment (1),the water-based solution containing the hexagonal ferrite precursor isheated and pressurized to a subcritical to supercritical state. Forexample, a water-based solution containing a hexagonal ferrite precursorcan be added to a liquid feed passage to which heated and pressurizedwater is being continuously fed to bring the water-based solution intocontact with the heated and pressurized water. In embodiment (2),because the hexagonal ferrite precursor can be instantaneously placed ina highly reactive state by bringing it into contact with heated andpressurized water, the conversion to ferrite can take place sooner,which is advantageous. Generally, water assumes a highly reactivesubcritical to supercritical state when heated to equal to or higherthan 200° C. and pressurized to equal to or higher than 20 MPa.Accordingly, in embodiment (2), the water is desirably heated to atemperature of equal to or higher than 200° C. and subjected to apressure of equal to or higher than 20 MPa.

Treating hexagonal ferrite magnetic particles with an organic modifyingagent is an effective means of preventing aggregation between particles.As is described in Japanese Unexamined Patent Publication (KOKAI) No.2009-208969, which is expressly incorporated herein by reference in itsentirety, in one embodiment, such an organic modifying agent can beadded to the reaction system once the conversion to ferrite has begun.

In another embodiment, before heating and pressurizing a water-basedsolution containing hexagonal ferrite magnetic particles and an Alcompound, described further below, the organic modifying agent can beadded to the water-based solution. The details are set forth furtherbelow.

In yet another embodiment, the organic modifying agent can be added tothe water-based solution containing the hexagonal ferrite precursor,after which the mixture is subjected to the process of embodiment (1) orembodiment (2) above. Thus, the organic modifying agent can adhere tothe hexagonal ferrite precursor particles, thereby effectivelypreventing the aggregation of particles and yielding an extremely finemagnetic powder.

Examples of the above organic modifying agent are organic carboxylicacids, organic nitrogen compounds, organic sulfur compounds, organicphosphorus compounds, salts thereof, surfactants, and various polymers.Examples of suitable polymers are those having a weight averagemolecular weight of about 1,000 to about 100,000. Those exhibiting watersolubility are desirable. Examples of desirable polymers are nonionicpolymers and hydroxyl group-comprising polymers. Salts of alkali metalsare suitable as the above salts. The above weight average molecularweight refers to a value that is measured by gel permeationchromatography (GPC) and converted to a polystyrene value.

Examples of organic carboxylic acids are aliphatic carboxylic acids,alicyclic carboxylic acids, and aromatic carboxylic acids. Aliphaticcarboxylic acids are desirable. The aliphatic carboxylic acid may be asaturated aliphatic carboxylic acid or an unsaturated aliphaticcarboxylic acid, with an unsaturated carboxylic acid being preferred.The number of carbon atoms of the carboxylic acid is not specificallylimited; for example, it can be equal to or more than 2. By way ofexample, it can be equal to or lower than 24, desirably equal to orlower than 20, preferably equal to or lower than 16. Specific examplesof aliphatic carboxylic acids are: oleic acid, linoleic acid, linolenicacid, caprylic acid, capric acid, lauric acid, behenic acid, stearicacid, myristic acid, palmitic acid, myristoleic acid, palmitoleic acid,vaccenic acid, eicosenoic acid, propanoic acid, butanoic acid, pentanoicacid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid,decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid,heptadecanoic acid, octadecanoic acid, nonadecanoic acid, icosanoicacid, and acetic acid; as well as dicarboxylic acids such as malonicacid, succinic acid, and adipic acid. However, there is no limitationthereto.

Examples of organic nitrogen compounds are organic amines, organic amidecompounds, and nitrogen-containing heterocyclic compounds.

The organic amine can be a primary amine, secondary amine, or tertiaryamine. Primary and secondary amines are desirable. Aliphatic amines arean example, as are primary and secondary aliphatic amines. The number ofcarbon atoms of the amines is not specifically limited; examples areequal to or more than 5 but equal to or lower than 24, desirably equalto or more than 8 and equal to or lower than 20, preferably equal to ormore than 12 but equal to or lower than 18. Specific examples of organicamines are alkylamines such as oleylamine, laurylamine, myristylamine,palmitylamine, stearylamine, octylamine, decylamine, dodecylamine,tetradecylamine, hexadecylamine, octadecylamine, and dioctylamine;aromatic amines such as aniline; hydroxyl group-comprising amines suchas methylethanolamine and diethanolamine; and derivatives thereof.

Examples of nitrogen-containing heterocyclic compounds are saturated andunsaturated heterocyclic compounds having three to seven-membered ringswith 1 to 4 nitrogen atoms. Hetero atoms in the form of sulfur atoms,oxygen atoms, and the like can be contained. Specific examples arepyridine, lutidine, cholidine, and quinolines.

Examples of organic sulfur compounds are organic sulfides, organicsulfoxides, and sulfur-containing heterocyclic compounds. Specificexamples are dialkyl sulfides such as dibutyl sulfide; dialkylsulfoxides such as dimethyl sulfoxide and dibutyl sulfoxide; andsulfur-containing heterocyclic compounds such as thiophene, thiolane,and thiomorpholine.

Examples of organic phosphorus compounds are phosphoric acid esters,phosphines, phosphine oxides, trialkyl phosphines, phosphorous acidesters, phosphonic acid esters, sub-phosphonic acid esters, phosphinicacid esters, and sub-phosphinic acid esters. Examples are tributylphosphine, trihexyl phosphine, trioctyl phosphine, and other trialkylphosphines; tributyl phosphine oxide, trihexyl phosphine oxide, trioctylphosphine oxide (TOPO), tridecyl phosphine oxide, and other trialkylphosphine oxides.

Examples of polymers and surfactants are polyethylene glycol,polyoxyethylene (1) lauryl ether phosphate, lauryl ether phosphate,sodium polyphosphate, sodium bis(2-ethylhexyl)sulfosuccinate, sodiumdodecylbenzene sulfonate, polyacrylic acid and its salts, polymethacryicacid and its salts, polyvinyl alcohol, other hydroxyl group-comprisingpolymers, polyvinyl pyrrolidone, other nonionic polymers, andhydroxyethyl cellulose. Any from among cationic, anionic, and nonionicsurfactants, as well as amphoteric surfactants, can be employed. Anionicsurfactants are desirable.

The above organic modifying agent is desirably admixed in a quantity ofabout 1 weight part to 1,000 weight parts per 100 weight parts ofhexagonal ferrite precursor. This makes it possible to more effectivelyinhibit particle aggregation. The organic modifying agent can be addedas is to the water-based solution containing the hexagonal ferriteprecursor, and addition of the organic modifying agent as a solution ina solvent (organic modifying agent solution) is desirable to obtainmagnetic powder of fine particles. A solvent in the form of water orwater and an organic solvent that is miscible with water or hydrophilicis desirable. From the above perspective, it is suitable to employ apolar solvent as the organic solvent. In this context, the term “polarsolvent” refers to a solvent that has a dielectric constant of equal toor higher than 15, has a solubility parameter of equal to or higher than8, or both. The various solvents set forth above are examples ofdesirable organic solvents.

The organic modifying agent can be mixed in batches or continuously.Doing so continuously with subsequent steps can enhance productivity, socontinuous mixing in is desirable.

FIG. 1 is a schematic diagram of a manufacturing device that is suitedto the continuous hydrothermal synthesis process. An example of aspecific embodiment of the method of manufacturing a magnetic powderaccording to an aspect of the present invention will be described basedon FIG. 1. However, the present invention is not limited to theembodiment set forth below.

The manufacturing device shown in FIG. 1 comprises liquid tanks 1, 2 and3; a heating means 4 (4 a to 4 c); pressurized liquid feeding means 5 a,5 b, and 5 c; a reaction flow passage 6; a cooling element 7; afiltering means 8; a pressure regulating valve 9; and a recovery element10. Fluid is fed from the various liquid tanks by pipes 100, 101, and102.

In one embodiment, water such as purified water or distilled water isintroduced to liquid tank 1, an aqueous solution containing hexagonalferrite precursor is introduced to liquid tank 2, and an organicmodifying agent solution is introduced to liquid tank 3. The water thatis introduced to liquid tank 1 is fed through pipe 100 while beingpressurized by pressurized liquid feeding means 5 a, and heated byheating means 4 to put the water in a subcritical to supercriticalstate, in which it arrives at mixing element M1.

The aqueous solution containing hexagonal ferrite precursor that hasbeen fed through pipe 101 by pressurized liquid feeding means 5 b fromliquid tank 2 converges with the organic modifying agent solution thathas been fed through pipe 102 by pressurized liquid feeding means 5 cfrom liquid tank 3, and arrives at mixing element M1.

Prior to arriving at mixing element M1, the organic modifying agentdesirably adheres to the surface of the hexagonal ferrite precursor. Itis advantageous in terms of obtaining fine particles of hexagonalferrite to make the organic modifying agent adhere to the hexagonalferrite precursor in this manner prior to bringing it into contact withthe water in a subcritical to supercritical state. To this end, water,or an organic solvent that is miscible with water or hydrophilic isdesirably employed as the solvent of the organic modifying agentsolution.

Next, the water-based solution containing the hexagonal ferriteprecursor is brought into contact with the water in a subcritical tosupercritical state in mixing element M1 to start converting theprecursor to ferrite. Subsequently, it is heated in reaction flowpassage 6 and further pressurized by pressurizing means 5a to cause thewater contained in the reaction system within reaction flow passage 6 toenter a subcritical to supercritical state and further advanceconversion of the precursor to ferrite. Subsequently, solutioncontaining hexagonal ferrite magnetic particles (M-type hexagonalstrontium ferrite containing Ca) obtained by the conversion of hexagonalferrite precursor to ferrite is discharged through discharge outlet M2.The discharged solution is cooled by mixing with cold water in coolingelement 7, after which the hexagonal ferrite magnetic particles arecollected by filtering means 8 (a filter or the like). The hexagonalferrite magnetic particles that have been collected by filtering means 8are released by filtering means 8, pass through pressure regulatingvalve 9, and are recovered in recovery element 10.

In the above method, since pressure is applied to the fluid that is fedto the interior, high-pressure metal pipe is desirably employed as thepiping. Due to low corrosion, a stainless steel such as SUS316 orSUS304, or a nickel-based alloy such as Inconel (registered trademark inJapan) or Hastelloy (registered trademark in Japan), is desirablyemployed as the metal constituting the piping. However, there is nolimitation to these materials, and an equivalent or similar material canalso be employed. Piping of the laminate configuration described inJapanese Unexamined Patent Publication (KOKAI) No. 2010-104928, which isexpressly incorporated herein by reference in its entirety, can also beemployed.

In the manufacturing device shown in FIG. 1, the water in a subcriticalto supercritical state and the water-based solution containing hexagonalferrite precursor are mixed together in a mixing element M1 where thepipes are joined by a T-shaped joint. However, it is also possible toemploy a reactor such as that employed in Japanese Unexamined PatentPublication (KOKAI) No. 2007-268503, 2008-12453, or 2010-75914, whichare expressly incorporated herein by reference in their entirety. Thereactor material is desirably that described in Japanese UnexaminedPatent Publication (KOKAI) No. 2007-268503, 2008-12453, or 2010-75914.Specifically, the metal constituting the piping is desirably one of theabove-described metals. However, it is not limited to them; anequivalent or similar material can be employed. Combination with alow-corrosion titanium alloy, tantalum alloy, ceramic, or the like isalso possible.

A process whereby an organic modifying agent is added to a water-basedsolution containing hexagonal ferrite precursor, after which the processof embodiment (2) above is conducted to obtain hexagonal ferritemagnetic particles modified with an organic modifying agent is describedabove. However, it is also possible to apply the process of embodiment(1) above after adding an organic modifying agent to the water-basedsolution containing the hexagonal ferrite precursor.

The reaction system in which water is present can be heated to equal toor higher than 300° C. and pressurized to equal to or higher than 20 MPato put the water in a subcritical to supercritical state to create areaction site of extremely high reactivity. Placing the hexagonalferrite precursor in this state can cause the conversion to ferrite toadvance rapidly, yielding hexagonal ferrite magnetic particles.

A heating temperature of equal to or higher than 300° C. within thereaction system will suffice, and the heating temperature falling withina range of 350° C. to 500° C. is desirable. The pressure that is appliedto the reaction system can be equal to or higher than 20 MPa as setforth above, desirably falling within a range of 20 MPa to 50 MPa.

In one embodiment, the above magnetic powder can contain Al, with atleast a part of the Al being present on the surface of the magneticpowder. To make Al adhere to the surface of the magnetic powder, it ispossible to apply the above hydrothermal synthesis process. For example,it is possible to make Al adhere to the surface of the hexagonal ferritemagnetic particles by heating and pressurizing a water-based solutioncontaining hexagonal ferrite magnetic particles and an Al compound. Toensure that the Al adhering treatment advances smoothly, the aboveheating and pressurizing are desirably conducted at a temperature andpressure at which water is in a subcritical to supercritical state. Asset forth above, heating a reaction system in which water is present toequal to or higher than 300° C. and pressurizing it to a pressure ofequal to or higher than 20 MPa will put the water in a subcritical tosupercritical state. Accordingly, the water-based solution containinghexagonal ferrite magnetic particles and an Al compound is desirablyheated to equal to or higher than 300° C. and pressurized to a pressureof equal to or higher than 20 MPa. The heating temperature preferablyranges from 350° C. to 500° C. and the pressure applied to the reactionsystem preferably ranges from 20 MPa to 50 MPa.

The above Al adhering treatment can be conducted in batches orcontinuously. From the perspective of enhancing productivity, the Aladhering treatment is desirably conducted continuously. It is preferablyconducted by continuously feeding the above water-based solution to areaction flow passage that heats the fluid flowing through it to equalto or higher than 300° C. and applies a pressure of equal to or higherthan 20 MPa. One example of a device that is suitable for conductingsuch a reaction is the manufacturing device described in FIG. 1 above.

An embodiment in which hexagonal ferrite magnetic particles to thesurface of which Al adheres by the manufacturing device shown in FIG. 1are obtained will be described next.

In FIG. 1, water is introduced into liquid tank 1. A solution containinghexagonal ferrite magnetic particles and an Al compound is introducedinto liquid tank 2. Water or a mixed solvent of water and an organicsolvent are examples of solvents employed in this solution. Examples oforganic solvents are the various solvents set forth above that aremiscible with water or hydrophilic. From the perspective of smoothprogression of the reaction, water is desirably employed as the solvent.

Examples of Al compounds are metal salts such as nitrates, sulfates, andacetates, as well as hydrates and metal alkoxides thereof. Metal saltsor hydrates that are highly soluble in water are desirably employed. Thequantity of Al compound that is employed can be a quantity thatincorporates a desired amount of Al into the magnetic powder beingprepared. To achieve smooth progression of the reaction, the quantity ofhexagonal ferrite magnetic particles in the above water-based solutionis desirably about 0.01 weight part to 10 weight parts per 100 weightparts of solvent.

To inhibit aggregation of hexagonal ferrite magnetic particles in thereaction and make Al adhere to fine particles, it is desirable to addthe organic modifying agent set forth above to the solution containingthe hexagonal ferrite magnetic particles and the Al compound. Theorganic modifying agent can be added as is to the solution containingthe hexagonal ferrite magnetic particles and the Al compound. Additionin the form of a solution of the organic modifying agent in solvent(organic modifying agent solution) is desirable from the perspective ofmaking Al adhere to fine particles of hexagonal ferrite magneticparticles. For example, the solution containing the hexagonal ferritemagnetic particles and the Al compound can be mixed with the organicmodifying agent solution by introducing the organic modifying agentsolution into liquid tank 3 in the manufacturing device shown in FIG. 1and causing pipe 102 to converge with pipe 101. Specific examples of theorganic modifying agent, desirable quantities employed, and detailsregarding solvents that can be used in the organic modifying agentsolution are as set forth above.

The following embodiments can be given as specific examples of theprocess of making Al adhere to the surface of the hexagonal ferritemagnetic particles.

(3) A water-based solution containing hexagonal ferrite magneticparticles and an Al compound, and optionally containing an organicmodifying agent, is continuously fed into a reaction flow passage inwhich the fluid flowing through it is heated to equal to or higher than300° C. and subjected to a pressure of equal to or higher than 20 MPa tomake Al adhere to the surface of the hexagonal ferrite magneticparticles in the reaction flow passage.

(4) A water-based solution containing hexagonal ferrite magneticparticles and an Al compound, and optionally containing an organicmodifying agent, is mixed with water that has been heated andpressurized, desirably heated to equal to or higher than 200° C. andpressurized to a pressure of equal to or higher than 20 MPa, after whichit is continuously fed into a reaction flow passage in which the fluidflowing through it is heated to equal to or higher than 300° C. andsubjected to a pressure of equal to or higher than 20 MPa to make Aladhere to the surface of the hexagonal ferrite magnetic particles in thereaction flow passage.

Embodiment (3) is a process that is similar to embodiment (1) set forthabove and embodiment (4) is a process that is similar to embodiment (2)set forth above. For example, it is possible to bring the water-basedsolution into contact with water that has been heated and pressurized byadding the water-based solution containing hexagonal ferrite magneticparticles and an Al compound to a liquid feed passage to which heatedand pressurized water is being continuously fed. Embodiment (4) differsfrom embodiment (3) in that the heated and pressurized water is broughtinto contact with the water-based solution containing the hexagonalferrite magnetic particles and an Al compound, while, in embodiment (3),the water-based solution containing the hexagonal ferrite magneticparticles and an Al compound is heated and pressurized to a subcriticalto supercritical state. In embodiment (4), because the Al compound canbe instantaneously placed in a highly reactive state by bringing it intocontact with heated and pressurized water, the Al adhering treatment cantake place sooner, which is advantageous. As set forth above, watergenerally assumes a highly reactive subcritical to supercritical statewhen it is heated to equal to or higher than 200° C. and pressurized toequal to or higher than 20 MPa. Accordingly, the heating andpressurizing of water in embodiment (4) are desirably conducted to atemperature of equal to or higher than 200° C. and a pressure of equalto or higher than 20 MPa.

For example, in mixing element M1 in the manufacturing device shown inFIG. 1, water in a subcritical to supercritical state is mixed with awater-based solution containing an Al compound and hexagonal ferritemagnetic particles, and optionally containing an organic modifyingagent. The mixed solution is heated and pressurized in reaction flowpassage 6 to cause the Al adhering treatment to progress. For example,the Al adhering treatment can take places by placing the Al compound ina highly reactive state to convert it to an oxide of Al (alumina).

A specific embodiment of a continuous hydrothermal synthesis process isas set forth above.

Some specific embodiments of the methods of manufacturing the magneticpowder for magnetic recording according to an aspect of the presentinvention have been described above. However, the magnetic powder formagnetic recording according to an aspect of the present invention isnot limited to that obtained by these manufacturing methods.

Magnetic Recording Medium

A further aspect of the present invention relates to a magneticrecording medium, which comprises a magnetic layer comprisingferromagnetic powder and binder on a nonmagnetic support, wherein theferromagnetic powder is the magnetic powder according to an aspect ofthe present invention as set forth above. The magnetic powder accordingto an aspect of the present invention can afford both a reduced size ofparticles and a high Ku, as well as high thermal stability in thehigh-density recording region. It can also exhibit a low SFD. Themagnetic recording medium according to an aspect of the presentinvention in which such magnetic powder is contained in a magnetic layeris suitable as a magnetic recording medium for high-density recording.

The magnetic recording medium according to an aspect of the presentinvention will be described in further detail below.

Details of the ferromagnetic powder that is employed in the magneticlayer, and of the method of manufacturing it, are as set forth above.

The magnetic layer contains ferromagnetic powder and binder.Polyurethane resins, polyester resins, polyamide resins, vinyl chlorideresins, acrylic resins such as those provided by copolymerizing styrene,acrylonitrile, methyl methacrylate and the like, cellulose resins suchas nitrocellulose, epoxy resins, phenoxy resins, polyvinylacetal,polyvinylbutyral, and other polyvinyl alkylal resins can be employedsingly, or as mixtures of multiple resins, as the binder contained inthe magnetic layer. Among these, desirable resins are polyurethaneresin, acrylic resins, cellulose resins, and vinyl chloride resins.These resins can also be employed as binders in the nonmagnetic layerdescribed further below. Reference can be made to paragraphs 0029 to0031 of Japanese Unexamined Patent Publication (KOKAI) No. 2010-24113,which is expressly incorporated herein by reference in its entirety,with regard to the above binders. Polyisocyanate curing agents can alsobe employed with the above resins.

Additives can be added as needed to the magnetic layer. Examples ofadditives are abrasives, lubricants, dispersing agents, dispersionadjuvants, antifungal agents, antistatic agents, oxidation inhibitors,solvents, and carbon black. The additives set forth above can besuitably selected for use from among commercial products based on theproperties desired.

Nonmagnetic layer

The contents of the nonmagnetic layer will be described in detail next.The magnetic recording medium of an aspect of the present invention cancomprise a nonmagnetic layer containing nonmagnetic powder and binderbetween the nonmagnetic support and the magnetic layer. The nonmagneticpowder that is employed in the nonmagnetic layer can be an organic or aninorganic material. Carbon black and the like can also be employed.Examples of inorganic materials are metals, metal oxides, metalcarbonates, metal sulfates, metal nitrides, metal carbides, and metalsulfides. Nonmagnetic powders of these materials are available ascommercial products and can be manufactured by known methods. Fordetails, reference can be made to Japanese Unexamined Patent Publication(KOKAI) No. 2010-24113, paragraphs 0036 to 0039.

The binders, lubricants, dispersing agents, additives, solvents,dispersion methods, and the like of the magnetic layer are also suitablefor use for the nonmagnetic layer. Techniques that are known formagnetic layers can also be applied to the quantity and type of binder,the quantities and types of additives and dispersing agents added, andthe like. Carbon black and organic material powders can also be added tothe nonmagnetic layer. In this regard, by way of example, reference canbe made to Japanese Unexamined Patent Publication (KOKAI) No.2010-24113, paragraphs 0040 to 0042.

Nonmagnetic Support

Examples of nonmagnetic supports are known supports such as biaxiallystretched polyethylene terephthalate, polyethylene naphthalate,polyamide, polyamide-imide, and aromatic polyamide. Among these,polyethylene terephthalate, polyethylene naphthalate, and polyamide aredesirable.

These supports can be subjected in advance to corona discharge, plasmatreatment, adhesion-enhancing treatment, heat treatment, or the like.The surface roughness of a nonmagnetic support that is suited to use inthe present invention is desirably a center average roughness Ra of 3 nmto 10 nm at a cutoff value of 0.25 mm.

Layer Structure

In the thickness structure of the magnetic recording medium according toan aspect of the present invention, the thickness of the nonmagneticsupport is desirably 3 μm to 80 μm. The thickness of the magnetic layercan be optimized based on the amount of saturation magnetization of themagnetic head employed, the length of the head gap, and the bandwidth ofthe recording signal. Generally, it can be 0.01 μm to 0.15 μm, desirably0.02 μm to 0.12 μm, and preferably, 0.03 μm to 0.10 μm. It suffices forthe magnetic layer to be comprised of a least one layer, but it canseparated into two or more layers having different magneticcharacteristics. The structures of known multilayer magnetic layers canbe applied.

The thickness of the nonmagnetic layer is for example 0.1 μm to 3.0 μm,desirably 0.3 μm to 2.0 μm, and preferably 0.5 μm to 1.5 μm. Thenonmagnetic layer of a magnetic recording medium according to an aspectof the present invention includes an essentially nonmagnetic layercontaining trace quantities of ferromagnetic powder, for example, eitheras impurities or intentionally, in addition to the nonmagnetic powder.The essentially nonmagnetic layer means a layer exhibiting a residualmagnetic flux density of equal to or less than 10 mT, a coercive forceof equal to or less than 7.96 kA/m (100 Oe), or a residual magnetic fluxdensity of equal to or less than 10 mT and a coercive force of equal toor less than 7.96 kA/m (100 Oe). The nonmagnetic desirably has noresidual magnetic flux density or coercive force.

Backcoat Layer

A backcoat layer can be provided on the opposite surface of thenonmagnetic support from that on which the magnetic layer is present inthe magnetic recording medium. The backcoat layer desirably comprisescarbon black and inorganic powder. The formula of the magnetic layer andnonmagnetic layer can be applied to the binder and various additivesused to form the backcoat layer. The thickness of the back coat layer isdesirably equal to or less than 0.9 μm, preferably 0.1 μm to 0.7 μm.

Manufacturing Method

The process of manufacturing the coating liquid for forming the magneticlayer, nonmagnetic layer, or backcoat layer normally comprises at leasta kneading step, dispersing step, and mixing steps provided as neededbefore and after these steps. The various steps can each be divided intotwo or more steps. All of the starting materials employed in the presentinvention, such as ferromagnetic powder, nonmagnetic powder, binder,carbon black, abrasives, antistatic agents, lubricants, and solvents,can be added either initially during the step or part way through. Anyindividual starting material can be divided for addition in two or moresteps. For example, polyurethane can be divided up and added during akneading step, dispersing step, or mixing step following dispersion toadjust the viscosity. In an aspect of the present invention,conventionally known manufacturing techniques can be employed for someof the steps. In the kneading step, it is desirable to employ anapparatus with powerful kneading strength in the kneading step, such asan open kneader, continuous kneader, pressurizing kneader, or extruder.Details on these kneading treatments are described in JapaneseUnexamined Patent Publication (KOKAI) Heisei No. 1-106338 and Heisei No.1-79274, which are expressly incorporated herein by reference in theirentirety. Glass beads can also be used to disperse the magnetic layercoating liquid, nonmagnetic layer coating liquid, or backcoat layercoating liquid. High specific gravity dispersing beads in the form ofzirconia beads, titania beads, and steel beads are also suitable. Theparticle diameter and packing rate of these dispersing beads can beoptimized for use. A known dispersing apparatus can be employed. Fordetails on methods of manufacturing magnetic recording media, referencecan be made to Japanese Unexamined Patent Publication (KOKAI) No.2010-24113, paragraphs 0051 to 0057, for example.

The magnetic recording medium according to an aspect of the presentinvention can afford both a high SNR and a low SFD by incorporating theabove magnetic powder in a magnetic layer.

EXAMPLES

The present invention will be described in greater detail below throughExamples. However, the present invention is not limited to theembodiments shown in Examples. The “parts” and “percent” indicated belowdenote “weight parts” and “weight percent,” respectively. Unlessspecifically stated otherwise, the steps and evaluations set forth belowwere conducted in air at 23° C.±1° C.

1. Preparation and Evaluation of the Magnetic Powder for MagneticRecording

<Preparing Magnetic Powder by the Glass Crystallization Method>

Example 1

A 1,725 g quantity of SrCO₃, 666 g of H₃BO₃, 1,332 g of Fe₂O₃, 52 g ofAl(OH)₃, and 34 g of CaCO₃ were weighed out and mixed in a mixer toobtain a starting material mixture.

The starting material mixture obtained was melted at a meltingtemperature of 1,380° C. in a platinum crucible. An outflow openingprovided in the bottom of the platinum crucible was heated whilestirring the melt, and the melt was discharged in rod form at about 6g/s. The discharged melt was quenched and rolled with a pair ofwater-cooled rolls to prepare an amorphous material.

A 280 g quantity of the amorphous material obtained was charged to anelectric furnace, heated to 645° C. (the crystallization temperature),and maintained for five hours at that temperature to cause hexagonalferrite particles to precipitate (crystallize).

Next, the crystallized product containing the hexagonal ferrite crystalswas coarsely crushed in a mortar. A 1,000 g quantity of 1 mm φ zirconiabeads and 800 mL of acetic acid were added. The mixture was dispersedfor three hours in a paint shaker. The dispersion was separated frombeads and put into a stainless beaker. The dispersion was then processedfor 3 hours at 100° C., precipitated in a centrifuge, and repeatedlydecanted to clean it. It was then dried for 6 hours at 110° C. to obtainparticles.

Example 2

With the exception that the quantity of CaCO₃ employed in preparing thestarting material mixture was 238 g, the same process was conducted asin Example 1.

Example 3

With the exception that the quantity of CaCO₃ employed in preparing thestarting material mixture was 136 g, the same process was conducted asin Example 1.

Example 4

With the exception that the quantity of CaCO₃ employed in preparing thestarting material mixture was 11 g, the same process was conducted as inExample 1.

Comparative Example 1

With the exception that no CaCO₃ was employed in preparing the startingmaterial mixture, the same process was conducted as in Example 1.

Comparative Example 2

With the exception that the quantity of CaCO₃ employed in preparing thestarting material mixture was 6 g, the same process was conducted as inExample 1.

Comparative Example 3

With the exception that the quantity of CaCO₃ employed in preparing thestarting material mixture was 272 g, the same process was conducted asin Example 1.

Example 5

With the exception that the quantity of Al(OH)₃ employed in preparingthe starting material mixture was 13 g, the same process was conductedas in Example 1.

Example 6

With the exception that the quantity of Al(OH)₃ employed in preparingthe starting material mixture was 35 g, the same process was conductedas in Example 1.

Example 7

With the exception that the quantity of Al(OH)₃ employed in preparingthe starting material mixture was 95 g, the same process was conductedas in Example 1.

Example 8

With the exception that the quantity of Al(OH)₃ employed in preparingthe starting material mixture was 6.2 g, the same process was conductedas in Example 1.

Example 9

With the exception that the crystallization temperature was changed to635° C., the same process was conducted as in Example 1.

Example 10

With the exception that the crystallization temperature was changed to630° C., the same process was conducted as in Example 1.

Example 11

With the exception that the crystallization temperature was changed to660° C., the same process was conducted as in Example 1.

Comparative Example 4

With the exception that the crystallization temperature was changed to670° C., the same process was conducted as in Example 1.

Comparative Example 5

With the exception that the crystallization temperature was changed to620° C., the same process was conducted as in Example 1.

Comparative Example 6

With the exception that 14 g of ZnO and 22 g of Nb₂O₅ were added inpreparing the starting material mixture, the same process was conductedas in Example 1.

Comparative Example 7

With the exception that 108.8 g of La₂O₃, 19 g of CoO, and 7 g of ZnOwere added and the quantity of CaCO₃ employed was 408 g in preparing thestarting material mixture, the same process was conducted as in Example1.

Example 12

With the exception that 57 g of BaCO₃ was added in preparing thestarting material mixture, the same process was conducted as in Example1.

Example 13

With the exception that 141 g of BaCO₃ was added in preparing thestarting material mixture, the same process was conducted as in Example1.

Example 14

With the exception that 76 g of BaCO₃ was added in preparing thestarting material mixture, the same process was conducted as in Example1.

Example 15

With the exception that 33 g of BaCO₃ was added in preparing thestarting material mixture, the same process was conducted as in Example1.

<Preparation of Magnetic Powder by Supercritical Synthesis Method>

Example 16 (1) Preparation of Precursor Aqueous Solution

In purified water were dissolved strontium nitrate (Sr(NO₃)₂.4H₂O),calcium nitrate (Ca(NO₃)₂.4H₂O, iron (III) nitrate (Fe(NO₃)₃.9H₂O), andKOH to prepare an aqueous solution (sol) of metal salts and metalhydroxides. The KOH was added in a quantity that yielded a reactionsolution pH of 9. The quantities of the various salts used to preparethe sol were adjusted to achieve a concentration in the aqueous solution(sol) prepared of 0.01 M and a Ca/Fe ratio of 0.5 atomic percent.

(2) Preparation of Modifying Agent Solution

Next, oleic acid was dissolved in ethanol to prepare a modifying agentsolution. The concentration of the solution prepared was 0.2 M.

(3) Synthesis of Hexagonal Ferrite

The aqueous solution (sol) was introduced into liquid tank 2 of themanufacturing device shown in FIG. 1 and the modifying agent solutionwas introduced into liquid tank 3. SUS316BA tube was used as the pipingin the manufacturing device.

Purified water that had been introduced into liquid tank 1 was heated byheater 4 while being fed by high pressure pump 5 a to causehigh-temperature, high-pressure water to flow through pipe 100. In thisprocess, the temperature and the pressure were controlled so that thetemperature of the high-temperature, high-pressure water after passingthrough heating means 4 c was 450° C. and the pressure was 30 MPa.

Additionally, the aqueous solution (sol) and modifying agent solutionwere fed to pipes 101 and 102 using high-pressure pumps 5 b and 5 c,respectively, at a liquid temperature of 25° C. so that the ratio byvolume of aqueous solution:modifying agent solution was 5:5. The twosolutions were mixed along the way. The mixed solution obtained was thenmixed with the high-temperature, high-pressure water in mixing elementMl. Subsequently, it was heated to 400° C. and pressurized to 30 MPa inreaction flow passage 6 to synthesize hexagonal ferrite magneticparticles.

Subsequently, the solution containing the hexagonal ferrite magneticparticles was cooled by cooling water in cooling element 7 and theparticles were collected.

The particles that were collected were washed with ethanol and thencentrifuged to separate the hexagonal ferrite magnetic particles thathad been modified with oleic acid.

(4) Al Adhering Treatment

The following operations were conducted to cause Al₂O₃ to precipitate bythe supercritical synthesis method on the surface of the hexagonalferrite magnetic particles obtained by the method set forth above.

Aluminum nitrate 9 hydrate (99.9% purity) was dissolved in pure water to0.1 M and hexagonal ferrite magnetic particles were admixed to an Al/Feratio of 4 atomic percent, yielding a mixed solution.

The mixed solution was introduced into liquid tank 2 of themanufacturing device shown in FIG. 1. The modifying agent solution wasintroduced into liquid tank 3 in the same manner as in (3) above.SUS316BA tube was employed as piping in the manufacturing device.

Purified water that had been introduced into liquid tank 1 was heated byheater 4 while being fed by high-pressure pump 5 a to causehigh-temperature, high-pressure water to flow through pipe 100. In thisprocess, the temperature and pressure were controlled so that thetemperature of the high-temperature, high-pressure water after passingthrough heating means 4 c was 450° C. and the pressure was 30 MPa.

Additionally, the mixed solution and the modifying agent solution werefed to pipes 101 and 102 at 25° C. by high-pressure pumps 5 b and 5 c,respectively, so that the ratio by volume of the mixedsolution:modifying agent solution was 50:50. The two were mixed alongthe way. The mixed solution obtained was mixed with thehigh-temperature, high-pressure water in mixing element Ml.Subsequently, it was heated to 450° C. and pressurized to 30 MPa inreaction flow passage 6, yielding hexagonal ferrite magnetic particlesto which Al₂O₃ adheres. The solution containing the particles was cooledwith cold water in cooling element 7 and the particles were collected.

Example 17

With the exception that the quantity of calcium nitrate (Ca(NO₃)₂.4H₂O)employed to prepare the precursor aqueous solution was changed so thatthe Ca/Fe ratio in the aqueous solution (sol) that was prepared was 3.0atomic percent, the same process was conducted as in Example 16.

Example 18

With the exception that the quantity of calcium nitrate (Ca(NO₃)₂.4H₂O)employed to prepare the precursor aqueous solution was changed so thatthe Ca/Fe ratio in the aqueous solution (sol) that was prepared was 1.5atomic percent, the same process was conducted as in Example 16.

Example 19

With the exception that barium nitrate (Ba(NO₃)₂) was added so as toachieve a Ba/Fe ratio in the aqueous solution (sol) that was prepared of1.5 atomic percent, the same process was conducted as in Example 16.

<Evaluation Methods>

(1) X-Ray Diffraction Analysis

When analyzing the particles obtained in Examples and ComparativeExamples by X-ray diffraction, the particles obtained in Examples 1 to15 and those obtained in Comparative Examples 2 to 7 were found tocomprise a single phase of M-type hexagonal ferrite.

The particles obtained in Comparative Example 1 were found to exhibit aCaFe₂O₄ peak in addition to an M-type hexagonal ferrite diffractionpeak.

Examples 16 to 19 were found to exhibit a y-type Al₂O₃ (alumina) peak inaddition to an M-type hexagonal ferrite diffraction peak.

In the X-ray diffraction analysis set forth below, CuKa radiation wasscanned under conditions of 45 kV and 40 mA and an XRD (X-raydiffraction) pattern was measured. The X-ray diffraction analysisspectra were measured under the following test conditions:

-   PANalytical X'Pert Pro diffractometer, PIXcel detectors-   Voltage 45 kV, intensity 40 mA-   Soller slits of incident beam and diffraction beam: 0.017 radians-   Fixed angle of dispersion slit: ¼ degree-   Mask: 10 mm-   Scattering prevention slit: ¼ degree-   Measurement mode: continuous-   Measurement time per stage: 3 seconds-   Measurement rate: 0.017 degree per second-   Measurement step: 0.05 degree

(2) Composition Analysis by ICP

A 0.01 g quantity of the magnetic powder obtained in each of Examplesand Comparative Examples was immersed in 10 mL of a 4N-HCl solution anddissolved by heating for 3 hours at 80 ° C. on a hot plate. The solutionwas diluted and then subjected to elemental analysis with an inductivelycoupled plasma (ICP) analysis device. The ratios of the various elementswere determined from the results.

(3) Measuring the Average Plate Diameter

The average particle size (average plate diameter) of the magneticpowders obtained in Examples and Comparative Examples was determined bythe method set forth above.

(4) Measurement of Saturation Magnetization σs

The saturation magnetization σs of each of the magnetic powders obtainedin Examples and Comparative Examples was measured at a field strength of1,194 kA/m (15 kOe) with a vibrating sample magnetometer (made byToei-Kogyo Co. Ltd.).

(5) Measurement of Anisotropy Constant Ku

Measurement was conducted at magnetic field sweep rates in the Hcmeasurement element of 3 minutes and 30 minutes with a vibrating samplemagnetometer (made by Toei-Kogyo Co. Ltd.), and anisotropy constant Kuwas calculated from the relational equation of Hc and magnetizationreversal volume due to thermal fluctuation given below. In the tablegiven further below, Ku is given in units of erg/cc. The valuesindicated in the table can be converted to the SI unit system based onthe conversion equation, 1 erg/cc=10⁻¹ J/m³.

Hc=2Ku/Ms(1−[(KuT/kV)1n(At/0.693)]½)

(In the equation, Ku: anisotropy constant; Ms: saturation magnetization;k: Boltzmann constant; T: absolute temperature; V: activation volume; A:spin precession frequency; t: magnetic field reversal time.)

(6) Measurement of SFD

The SFD of each of the magnetic powders obtained in Examples andComparative Examples was calculated by using a vibrating samplemagnetometer (mode by Toei-Kogyo Co., Ltd.) to apply a magnetic field toa magnetic powder sample, taking the half width of the differentialcurve (dσ/dH) on the magnetization curve of the second quadrant, anddividing by the coercive force Hc value of the magnetic powder.

(7) Confirming the State of the Al

Observation of the cross-sections of particles of the magnetic powderprepared in Examples 1 to 15 by high-resolution TEM revealed theformation of a coating on the outer surface thereof.

Further, in analysis of the magnetic powders prepared in Examples 1 to15 by X-ray photoemission spectroscopy (XPS), measurement of the Al/Feratio at a depth of about 0.5 nm from the particle surface revealed avalue of 1.5 to 2.0-fold the value measured by ICP in (2) above,confirming the localization of Al in the outer layer.

Based on the above results, the presence of a coating of Al on thesurface of the particles was confirmed for the magnetic powder preparedin Examples 1 to 15. The above TEM observation revealed the formation ofa coating on primary particles in the magnetic powders obtained inExamples 1 to 15.

2. Preparation and Evaluation of a Magnetic Recording Medium (MagneticTape)

Magnetic tapes were prepared by the following method using the magneticpowder prepared in Examples and Comparative Examples.

(1) Formula of Magnetic Layer Coating Liquid

Hexagonal ferrite magnetic powder: 100 parts Polyurethane resin: 12parts Weight average molecular weight: 10,000 Content of sulfonic acidfunctional groups: 0.5 meq/g Diamond particles (50 nm average particlediameter): 2 parts Carbon black (#55, made by Asahi Carbon, particles0.5 part size 0.015 μm): Stearic acid: 0.5 part Butyl stearate: 2 partsMethyl ethyl ketone: 180 parts Cyclohexanone: 100 parts

(2) Nonmagnetic Layer Coating Liquid

Nonmagnetic powder: α-iron oxide: 100 parts Average primary particlediameter: 0.09 μm Specific surface area by BET method: 50 m²/g pH: 7 DBPoil absorption capacity: 27 to 38 g/100 g Surface treatment agent Al₂O₃:8 weight percent Carbon black (Conductex SC-U made by Columbia 25 partsCarbon): Vinyl chloride copolymer (MR104, made by Zeon 13 partsCorporation): Polyurethane resin (UR8200, made by Toyobo): 5 partsPhenyl phosphonic acid: 3.5 parts Butyl stearate: 1 part Stearic acid: 2parts Methyl ethyl ketone: 205 parts Cyclohexanone: 135 parts

(3) Preparation of Magnetic Tape

For each of the various coating liquids set forth above, the variouscomponents were kneaded in a kneader. The liquid was passed by pumpthrough a sand mill in which had been introduced a quantity of 1.0 mm(I) zirconia beads that filled 65 percent of the volume of thedispersion part and dispersing process was conducted for 120 minutes(the period of actual residence in the dispersion part) at 2,000 rpm. Tothe dispersion obtained were added 6.5 parts of polyisocyanate in thecase of the nonmagnetic layer coating liquid. Another 7 parts of methylethyl ketone were added. The mixture was filtered with a filter havingan average pore diameter of 1 μm to prepare a coating liquid for forminga nonmagnetic layer and a coating liquid for forming a magnetic layer.

The nonmagnetic layer coating liquid obtained was coated and dried to athickness of 1.0 μm on a polyethylene naphthalate base 5 μm inthickness, followed by a magnetic layer 70 nm in thickness in sequentialmultilayer coating. After drying, the product was processed at a linearpressure of 300 kg/cm at a temperature of 90° C. in a seven-stagecalender. The product was slit to a width of ¼ inch and the surface waspolished to obtain a magnetic tape.

<Evaluation Methods>

Evaluation of SNR

A recording head (MIG, gap 0.15 μm, 1.8 T) and a reproduction GMR headwere mounted on a drum tester, signals were recorded at a track densityof 16 KTPI and a linear recording density of 400 Kbpi (surface recordingdensity of 6.4 Gbpsi), and the reproduction output, noise, and SNR ofeach of the magnetic tapes were measured. The value measured forComparative Example 1 was employed as a reference.

The results of the above are given in the following table.

TABLE 1 Ca/(Sr + Ca) Ba/(Sr + Ca + Ba) Synthesis method Ca/Fe at % at %Ba/Fe at % at % Al/Fe at % Ex. 1 Glass crystallization method 0.4 4 — —4.2 Ex. 2 Glass crystallization method 2.8 18 — — 4.1 Ex. 3 Glasscrystallization method 1.5 10 — — 4.2 Ex. 4 Glass crystallization method0.08 2 — — 4.0 Comp. Ex. 1 Glass crystallization method — — — — 4.7Comp. Ex. 2 Glass crystallization method 0.04 1 — — 4.0 Comp. Ex. 3Glass crystallization method 3.2 22 — — 4.3 Ex. 5 Glass crystallizationmethod 0.4 4 — — 1.0 Ex. 6 Glass crystallization method 0.4 4 — — 2.8Ex. 7 Glass crystallization method 0.4 4 — — 7.8 Ex. 8 Glasscrystallization method 0.4 4 — — 0.4 Ex. 9 Glass crystallization method0.4 4 — — 4.1 Ex. 10 Glass crystallization method 0.4 4 — — 4.2 Ex. 11Glass crystallization method 0.4 4 — — 4.1 Comp. Ex. 4 Glasscrystallization method 0.4 4 — — 4.3 Comp. Ex. 5 Glass crystallizationmethod 0.4 4 — — 4.1 Comp. Ex. 6 Glass crystallization method 0.4 4 — —4.1 Comp. Ex. 7 Glass crystallization method 5.0 42 — — — Ex. 12 Glasscrystallization method 0.4 4 1.9 12 4.1 Ex. 13 Glass crystallizationmethod 0.4 4 4.8 38 4.0 Ex. 14 Glass crystallization method 0.4 4 3.3 204.2 Ex. 15 Glass crystallization method 0.4 4 1.2  6 4.3 Ex. 16Supercritical synthesis method 0.4 4 — — 3.8 Ex. 17 Supercriticalsynthesis method 2.6 15 — — 3.6 Ex. 18 Supercritical synthesis method1.4 9 — — 3.6 Ex. 19 Supercritical synthesis method 0.4 4 1.7 10 3.6Presence or Average absence of rare plate earth elements, diametertransition metal Analysis result by Ku * 10E6 nm elements X-raydiffraction analysis σs Am²/kg erg/cc SFD SNR dB Ex. 1 18 —Magnetoplumbite structure 52 2.1 1.0 +0.9 Ex. 2 17 — Magnetoplumbitestructure 55 2.3 0.9 +1.0 Ex. 3 17 — Magnetoplumbite structure 53 2.20.9 +0.8 Ex. 4 18 — Magnetoplumbite structure 51 2.1 1.1 +0.9 Comp. Ex.1 20 — Magnetoplumbite structure 45 1.8 1.3 +0.0 Comp. Ex. 2 18 —Magnetoplumbite structure 48 1.9 1.3 −0.1 Comp. Ex. 3 19 —Magnetoplumbite structure, 40 1.6 2.0 −3.0 CaFe₂O₄ Ex. 5 17 —Magnetoplumbite structure 55 2.1 1.1 +1.1 Ex. 6 20 — Magnetoplumbitestructure 54 2.1 1.0 +1.0 Ex. 7 16 — Magnetoplumbite structure 46 2.21.5 +0.1 Ex. 8 19 — Magnetoplumbite structure 55 1.9 0.8 +0.2 Ex. 9 14 —Magnetoplumbite structure 52 2.0 1.2 +1.2 Ex. 10 11 — Magnetoplumbitestructure 51 2.0 1.3 +1.3 Ex. 11 23 — Magnetoplumbite structure 54 2.30.8 +0.6 Comp. Ex. 4 27 — Magnetoplumbite structure 56 2.4 0.7 −0.1Comp. Ex. 5 9 — Magnetoplumbite structure 47 1.7 1.9 −0.8 Comp. Ex. 6 19Zn, Nb Magnetoplumbite structure 42 1.6 2.3 −0.5 Comp. Ex. 7 12 La, Co,Zn Magnetoplumbite structure 46 1.8 2.5 −0.8 Ex. 12 19 — Magnetoplumbitestructure 50 2.0 0.8 +1.3 Ex. 13 18 — Magnetoplumbite structure 50 2.00.6 +1.4 Ex. 14 20 — Magnetoplumbite structure 51 2.1 0.7 +1.3 Ex. 15 20— Magnetoplumbite structure 49 2.0 0.8 +1.3 Ex. 16 14 — Magnetoplumbitestructure, Al₂O₃ 55 2.3 0.9 +1.2 Ex. 17 13 — Magnetoplumbite structure,Al₂O₃ 56 2.5 0.8 +1.1 Ex. 18 14 — Magnetoplumbite structure, Al₂O₃ 552.4 0.8 +1.0 Ex. 19 14 — Magnetoplumbite structure, Al₂O₃ 53 2.2 0.6+1.4

Evaluation Results

As shown in Table 1, the magnetic powders of Examples exhibited a highKu and a low SFD. Magnetic tapes prepared using the above magneticpowders exhibited good electromagnetic characteristics (SNR). Oneembodiment of the present invention could yield high Ku, low SFDmagnetic powder exhibiting, for example, a Ku of equal to or higher than2.0×10⁶ erg/cc (for example, 2.0×10⁶ erg/cc to 3.0×10⁶ erg/cc) and anSFD of equal to or lower than 1.2 (for example, 0.5 to 1.2).

As shown in Table 1, the magnetic powders containing prescribedquantities of Ca of Examples exhibited high saturation magnetization σs.Among them, the magnetic powders of Examples 1 to 6 and 8 to 19, whichcontained prescribed quantities of Al in addition to Ca, exhibitedmarkedly higher σs.

Based on the above results, an aspect of the present invention could bedetermined to provide magnetic powder for magnetic recording that iscomprised of fine particles, afforded a high Ku and a low SFD, and wassuited to high-density recording, as well as achieved a high σs.

An aspect of the present invention is useful in the field ofmanufacturing magnetic recording media for high-density recording.

Although the present invention has been described in considerable detailwith regard to certain versions thereof, other versions are possible,and alterations, permutations and equivalents of the version shown willbecome apparent to those skilled in the art upon a reading of thespecification and study of the drawings. Also, the various features ofthe versions herein can be combined in various ways to provideadditional versions of the present invention. Furthermore, certainterminology has been used for the purposes of descriptive clarity, andnot to limit the present invention. Therefore, any appended claimsshould not be limited to the description of the preferred versionscontained herein and should include all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

Having now fully described this invention, it will be understood tothose of ordinary skill in the art that the methods of the presentinvention can be carried out with a wide and equivalent range ofconditions, formulations, and other parameters without departing fromthe scope of the invention or any Examples thereof.

All patents and publications cited herein are hereby fully incorporatedby reference in their entirety. The citation of any publication is forits disclosure prior to the filing date and should not be construed asan admission that such publication is prior art or that the presentinvention is not entitled to antedate such publication by virtue ofprior invention.

What is claimed is:
 1. Magnetic powder, which is magnetoplumbitehexagonal strontium ferrite magnetic powder comprising 0.05 atomicpercent to 3 atomic percent of Ca per 100 atomic percent of Fe, butcomprising no rare earth elements or transition metal elements otherthan Fe, the average particle size of which ranges from 10 nm to 25 nm,and which is magnetic powder for magnetic recording.
 2. The magneticpowder according to claim 1, wherein a content of Ca per 100 atomicpercent of a combined content of Sr and Ca ranges from 1 atomic percentto 20 atomic percent.
 3. The magnetic powder according to claim 1, whichfurther comprises Al.
 4. The magnetic powder according to claim 3,wherein a content of Al per 100 atomic percent of Fe ranges from 0.5atomic percent to 6 atomic percent.
 5. The magnetic powder according toclaim 3, wherein at least a part of the Al is present on a surface ofthe magnetic powder.
 6. The magnetic powder according to claim 1, whichfurther comprises Ba.
 7. The magnetic powder according to claim 6,wherein a content of Ba per 100 atomic percent of a combined content ofSr, Ca, and Ba ranges from 5 atomic percent to 40 atomic percent.
 8. Themagnetic powder according to claim 1, which comprises 0.5 atomic percentto 6 atomic percent of Al per 100 atomic percent of Fe and 5 atomicpercent to 40 atomic percent of Ba per 100 atomic percent of a combinedcontent of Sr, Ca, and Ba.
 9. The magnetic powder according to claim 8,wherein at least a part of the Al is present on a surface of themagnetic powder.
 10. A magnetic recording medium, which comprises amagnetic layer comprising ferromagnetic powder and binder on anonmagnetic support, wherein the ferromagnetic powder is the magneticpowder according to claim
 1. 11. A method of manufacturing magneticpowder for magnetic recording, which comprises providing the magneticpowder for magnetic recording according to claim 1 by a glasscrystallization method with a starting material mixture comprising atleast Sr, Ca, and Fe.
 12. The method of manufacturing magnetic powderfor magnetic recording according to claim 11, wherein the startingmaterial mixture further comprises Al.
 13. The method of manufacturingmagnetic powder for magnetic recording according to claim 11, whereinthe starting material mixture further comprises Ba.
 14. A method ofmanufacturing magnetic powder for magnetic recording, wherein themagnetic powder for magnetic recording medium is the magnetic powder formagnetic recording according to claim 1 , and the method comprises:preparing a hexagonal ferrite precursor by mixing an Fe salt, an Srsalt, and a Ca salt in a base-containing water-based solution; andconverting the hexagonal ferrite precursor to hexagonal ferrite byfeeding a water-based solution comprising the hexagonal ferriteprecursor that has been prepared continuously to a reaction flow passagewhile heating the water-based solution to a temperature of equal to orhigher than 300° C. as well as applying a pressure of equal to or higherthan 20 MPa to the water-based solution.
 15. The method of manufacturingmagnetic powder for magnetic recording according to claim 14, whereinthe conversion to hexagonal ferrite is conducted by: mixing thewater-based solution comprising the hexagonal ferrite precursor that hasbeen prepared with an organic modifying agent; then mixing a solution,that has been provided by the mixing, with water that is beingcontinuously fed while heating and applying pressure to prepare amixture and feeding the mixture to the reaction flow passage.
 16. Themethod of manufacturing magnetic powder for magnetic recording accordingto claim 14, which further comprises heating and pressurizing awater-based solution comprising an Al compound and hexagonal ferritethat has been provided by the conversion of the hexagonal ferriteprecursor to make Al adhere to a surface of the hexagonal ferrite. 17.The method of manufacturing magnetic powder for magnetic recordingaccording to claim 16, wherein the heating and pressurizing is conductedby feeding the water-based solution, comprising an Al compound andhexagonal ferrite that has been prepared by the conversion of thehexagonal ferrite precursor, continuously to a reaction flow passage thefluid flowing through which is heated to equal to or higher than 300° C.and pressurized to a pressure of equal to or higher than 20 MPa.
 18. Themethod of manufacturing magnetic powder for magnetic recording accordingto claim 17, which further comprises mixing the water-based solutioncomprising an Al compound and hexagonal ferrite provided by theconversion of the hexagonal ferrite precursor with water that iscontinuously fed while being heated and pressurized, and then feeding tothe reaction flow passage.
 19. The method of manufacturing magneticpowder for magnetic recording according to claim 14, which furthercomprises mixing a Ba salt with an Fe salt, Sr salt, and Ca salt duringthe mixing in a base-containing water-based solution.